Amorphous fix

[saransh13]

Two fixes:

1. Sometimes the simulated spectra has some nans in it. The wppf.DOC property was being calculated as nans for these cases. Fixing it.
2. the amorphous_lineout for experimental spectra with multiple phases was initialized to zeros inside the loop. Fixing that.

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.26%. Comparing base (314c567) to head (36d2865).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/wppf/WPPF.py	18.18%	9 Missing ⚠️
hexrd/wppf/amorphous.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #830      +/-   ##
==========================================
- Coverage   49.26%   49.26%   -0.01%     
==========================================
  Files         144      144              
  Lines       23299    23306       +7     
==========================================
+ Hits        11479    11481       +2     
- Misses      11820    11825       +5     

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